A fuel cell is a power source that converts chemical energy into electrical energy by exploiting the oxidation of a fuel. The fuel, generally hydrogen or methanol, is contained in a cartridge that can be substituted in a few seconds.
PEMFCs are particular types of fuel cells (Polymer Electrolyte Membrane Fuel Cell) that function at low temperatures and which therefore are well suited for a quickly growing market like that of portable applications.
The functioning of a PEMFC is essentially assured by two electrodes, anode and cathode, where the electrochemical reactions take place that generate electrical energy, and by an electrolyte which transports the ions from the anode to the cathode, which in the specific case comprising the aforesaid polymer membrane.
In the case of hydrogen fuel cells, the reactions involved are:H2→2H++2e− (anode)½O2+2e−+2H+→H2O (cathode)
while in the case of direct methanol fuel cells, the following reactions take place:CH3OH+H2O→6e−+6H++CO2 (anode)3/2O2+6e−+6H+→3H2O (cathode)
FIG. 1 shows a typical diagram of the assembly comprising the electrodes and the polymer electrolyte membrane of a PEMFC, commonly defined MEA or “Membrane Electrode Assembly.”
Such a polymer electrolyte membrane, or polyelectrolyte membrane, is positioned between the electrodes of the cell and has the function of electronically isolating the anode and cathode, allowing however the protons developed in the anode to pass through.
In turn, the electrons developed at the anode are provided to an external load, to then be consumed together with the protons once they have reached the cathode.
An electrode is typically formed by a catalytic layer where the related electrochemical reaction is triggered, and by a diffusive layer which supports the catalytic layer and which acts as a collector of the electrons participating in the electrode reactions in addition to acting as a diffuser of the chemical reagents, that is, as a diffuser of the fuel and oxygen that participate in the aforesaid reactions.
The catalytic layer may therefore favor the transport of the reagents, the ionic one (proton) and the electronic one, which occurs in respective so-called porous phase, electrolyte phase or proton transport phase, and electronically conductive phase or electron transport phase, the latter being the phase that supports the catalyst.
The catalytic layer may therefore also favor the simultaneous contact between the aforesaid three phases. Regarding the catalyst, it should be added that it may advantageously have a good efficiency, i.e. the catalyst may have a high dispersion coefficient, which is defined as the ratio between the number of surface atoms and the total number of atoms.
The higher the ratio, the more efficiently the catalyst is used, since the surface area on which the electrochemical reactions can take place is greater. A known process for making the catalytic layer involves the deposition of a catalyst ink comprising a proton transport polymer phase, a catalyst supported by electronically conductive particles, and possibly pore-forming agents for the creation of porosity, in addition to additives of a binder type to favor a continuity in the contact between the proton transport phase and the electron transport phase.
In turn, the proton transport phase is generally formed by the same material of the electrolyte membrane to favor, in this case, the continuity of such phase at the interface between the catalytic layer and the polyelectrolyte membrane. Once again, with regard to the catalyst, it should be said that the anode electrode and the cathode electrode are generally made with metals of a different nature. For the cathode electrode, the preferred catalytic metals are platinum and alloys of such metal with cobalt or chromium, while ruthenium, rhodium, iridium, palladium, platinum and their alloys are preferred for the anode electrode. Particularly in the case of the methanol fuel cells, the anode catalyst preferably comprises platinum or iridium alloys.
The electron transport phase, supporting the catalyst, is generally carbon-black (CB), and may contain organic groups on the surface, which favor their dispersion inside the polymer phase. The pore-forming agent can be any polymer which is soluble and/or removable via heat treatment (for example polysaccharides, polyethylene glycols etc.), or it can be a salt or a mixture of salts that can be removed via washing, like carbonates. As binder, polyolefins can be used, like polyethylene or polypropylene and other polymers like polyesters, polycarbonates, polyimides and the like.
As an electronically conductive load for PEMFC, the prior art has also provided the use of conductive polymers supporting the catalyst. The interest towards electronically conductive polymers arises from the higher conductivity which they have with respect to Carbon black (CB). The electronic conductivity of the Carbon black varies, for example, between 10−2 and 10−1 S/cm, while that of the conductive polymers can be up to 102 S/cm.
Studies have also been conducted on catalytic activity confirming that, the catalyst used being the same, the capacity to activate the electrochemical reactions is increased when conductive polymers are used as supports. For example, the catalytic activity of the platinum dispersed on the polyaniline (PANI) towards the oxidation of the methanol is greater than that of the platinum dispersed on Carbon black, as is described in Akira Kitani, Tetsuro Akashi et al., “Electrocatalytic oxidation of methanol on platinum modified polyaniline electrodes” Synthetic Metals 121 (2001) 1301-1302.
Analogously, platinum particles dispersed on the polyaniline favor the oxygen reduction reaction more than the platinum supported on Carbon black does, as described for example in Evelyn K. W. Lai, et al., “Electrocatalytic Reduction of Oxygen by Platinum Microparticles Deposited on Polyaniline Films”, Synthetic Metals 84 (1997), 87-88.
One study conducted on polyaniline moreover demonstrates that such polymer is capable of oxidizing the methanol even in the absence of catalyst, nevertheless in this case the power produced from the fuel cell is very low.
In any case, in each of the different studied conditions, it was shown that the increased catalytic activity permits reducing the quantity of catalyst, giving the same performance—the catalyst being a costly part in the making of the catalytic layer.
Generally, in the case of support such as Carbon black, the catalyst is deposited on the surface of the Carbon black either by chemical reduction or by electrodeposition, while if the electronically conductive phase comprises a polymer of the above-considered type, it is possible to incorporate the catalyst inside the polymer support.
In this second case, it is the entire three-dimensional structure of the polymer that acts as support for the catalyst, with consequent advantage of an increased overall catalytic capacity. The conductive polymers, in addition to the above-described advantages, can also improve the humidification conditions of the electrode due to the presence of different atoms along the chain, such as nitrogen (N), oxygen (O) and sulphur (S) atoms; this characteristic is enhanced in the case of conductive polymers which are sulfonates.
Improving the humidification of the electrode signifies reducing the electrical contact resistance, linked to the exchange of protons from the catalyst surface to the proton transport phase. In this manner, the loss of electrical power following such phenomenon is reduced, improving the efficiency of the entire fuel cell.
There is therefore a growing interest towards electronically conductive polymers supporting redox catalysts, in particular, for making catalytic layers of fuel cell electrodes, which can be an increasingly valid and improved alternative to the use of Carbon black.